Dart Lambda / Shortland函数混淆 [英] Dart lambda/shortland function confusion

问题描述

我对Dart还是很陌生，=>（胖箭头）的语法仍然让我感到困惑（我来自C＃背景）。

所以在C＃中粗箭头（=>）说：进入，例如：

  Action< string> action1 =（str）=> {System.Diagnostic.Debug.WriteLine（接收的参数： + str.ToString（））; } 
 
 action1（ Some parameter）; 
  

是指：作为参数发送给 action1 的任何内容（如果可以将其强制转换为 string ）转到内部范围（在我们的示例中，它只是打印在 Debug.WriteLine中（）

，但在Dart中则有所不同....（？）

例如在 Future.then

中

  ClassWithFutures myClass = new ClassWithFutures（）; 
 myClass.loadedFuture.then（
（str）=> {print（类已加载信息：$ str），
 onError ：（exp）=> {print（在类加载中发生错误。错误是：$ exp）;} 
）; 
  

Dart编辑器警告我，第一和第二张打印是：地图输入键的预期字符串文字。我认为在C＃中， str 只是为将由 Future.then 用于调用 onValue 或 onError

我在做什么错了吗？

解决方案

您需要选择块语法或单表达式语法，但不能全部选择。

您不能将=>与{}

您的两个示例如下所示：

  ClassWithFutures myClass = new ClassWithFutures（）; 
 myClass.loadedFuture.then（
（str）=> print（类已加载信息：$ str），
 onErrro：（exp）=> print（ Error错误是：$ exp）
）; 
  

或

  ClassWithFutures myClass = new ClassWithFutures（）; 
 myClass.loadedFuture.then（
（str）{print（类已加载信息：$ str）;}，
 onErrro：（exp）{print（发生错误错误是：$ exp）;} 
）; 
  

在两种情况下，这都是表达匿名函数的一种方式。

通常，如果您只想运行一个表达式，则可以使用=>语法来获得更整洁的代码。示例：

  someFunction.then（（String str）=> print（str））; 
  

或者您可以使用带有大括号的块语法来完成更多工作，或者使用单个表达式。

  someFunction.then（（String str）{
 str = str +你好世界； 
 print（str）; 
}）； 
  

但是您不能将它们组合在一起，因为这时您将使用两种函数创建语法，但会中断。 / p>

希望这会有所帮助。

I'm still pretty new to Dart and the syntax of => (fat arrow) still confuses me (I come from C# background).

So in C# fat arrow ( => ) says: goes to so for example:

Action<string> action1 = (str) => { System.Diagnostic.Debug.WriteLine("Parameter received: " + str.ToString()); }

action1("Some parameter");

means: whatever send as parameter to action1 (if it could be casted to string) goes to inner scope (in our case it just printed in Debug.WriteLine()

but in Dart it's something different.... (?)

for example in Future.then

ClassWithFutures myClass = new ClassWithFutures();
myClass.loadedFuture.then( 
   (str) => { print("Class was loaded with info: $str"),
   onError: (exp) => { print("Error occurred in class loading. Error is: $exp"); }
);

Dart editor warn me that the first and second print is: Expected string literal for map entry key. I think in C# way that str it just name for parameter that will be filled by internal callback that Future.then uses to call onValue or onError

What I'm doing wrong ?

解决方案

You need to choose either block syntax or single expression syntax, but not both.

You can't combine => with {}

Your two options are as follows using your example:

ClassWithFutures myClass = new ClassWithFutures();
myClass.loadedFuture.then( 
  (str) => print("Class was loaded with info: $str"),
  onErrro: (exp) => print("Error occurred in class loading. Error is: $exp")
);

or

ClassWithFutures myClass = new ClassWithFutures();
myClass.loadedFuture.then( 
  (str) { print("Class was loaded with info: $str"); },
  onErrro: (exp) { print("Error occurred in class loading. Error is: $exp"); }
);

In both cases, it is just a way to express an anonymous function.

Normally if you want to just run a single expression, you use the => syntax for cleaner and more to the point code. Example:

someFunction.then( (String str) => print(str) );

or you can use a block syntax with curly braces to do more work, or a single expression.

someFunction.then( (String str) {
  str = str + "Hello World";
  print(str);
});

but you can't combine them since then you are making 2 function creation syntaxes and it breaks.

Hope this helps.

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